A related art light guide plate has been known to receive the incident light entered from an end surface thereof to emit the light from both main surfaces thereof. For example, Patent Document 1 discloses such a related art light guide plate for use in an illuminating device.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-299716
Particularly, the illuminating device disclosed in Patent Document 1 includes the light guide plate, a first display panel, and a second display panel. The first and second display panels are disposed in such a manner as to sandwich the light guide plate therebetween. The illuminating device allows the incident light entered into the light guide plate from a light source, such as a light emitting diode (LED) disposed on the end surface of the light guide plate, to be emitted from the both main surfaces of the light guide plate, so that the light is irradiated from back of the first and second display panels.
In a case where such an illuminating device is applied to a relatively large device such as a liquid crystal monitor and a light emitting sign, however, an amount of the light emitted from the light guide plate is not sufficient, causing difficulty of fulfilling a function thereof. As a method for increasing the amount of the light to be emitted from such a light guide plate, a method has been known for forming a reflection pattern on a light guide plate by an ultrasonic process as an invention set forth in Patent Document 2. In a case of forming the reflection pattern, an ultrasonic process horn is attached to a prescribed ultrasonic processing machine, so that a light guide plate workpiece is allowed to contact the ultrasonic process horn for plural times to form the reflection pattern.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-66699